Friction materials used in automotive pads have certain friction, wear and noise characteristics. The ideal pad would produce a constant friction at all operating temperatures, would operate noiselessly, and would experience negligible wear throughout a nearly infinite life. This is the holy grail of friction material formulators. Of course, such a utopian material has yet to be discovered. Until then, formulators continually try to optimize these three characteristics: friction uniformity (low "brake fade"), good wear (long life) and low noise (attenuation) by creating new friction material formulations.
Conventional friction material pads are produced by dry blending friction and lubricating particles, fibers and fillers in a resin matrix, forming into pucks and then heating under pressure in a mold to produce pads, which are bonded to backing plates for use in brakes, clutches and the like. These pads are generally physically homogeneous and thus have uniform density, hardness, compressibility, dampening and wear resistance throughout. During the molding process, the applied pressure causes most of the fibers to orient generally parallel to the wear face of the pad. When the pad is subject to use, friction produces heat which tends to cause the fibers to be pried out of the matrix since they lie nearly wholly on the surface of the friction face. This causes a degradation of friction characteristics and accelerates pad wear.
Patent application Ser. No. 08/922436, filed Sep. 2, 1997 and issued as U.S. Pat. No. 5,894,049 on Apr. 13, 1999 discloses a process which addresses this problem by forming a length of paper that has a majority of the fibers oriented generally parallel to the length of the paper. The paper is then folded or cut into strips which are laminated, with the edges of the paper strips oriented perpendicular to the pad wear surface and perpendicular to the direction of relative rotation of the pad and its mating friction surface. This arrangement provides a pad having wear characteristics superior to conventional pads because the fibers, platelets, flakes or the like are imbedded in the material matrix perpendicular to the friction face, and cannot be pried out and lost.
The outer diameter, or edge of a friction material pad wears faster than the inner diameter, or edge, due to a higher relative linear velocity between the pad and its opposing friction surface, usually a brake disk. To overcome this unequal wear problem, a friction material pad has been developed which comprises a lamination of several stacks of sheets, each of which has a different composition. All of the sheets have a top edge forming the friction face that is generally parallel to the direction of relative rotation between the friction pad and opposing rotary member. Each sheet has a plurality of unwoven fibers, more than half of which are oriented within 45.degree. of perpendicularity to the top edge. The sheets are bound together by a resin binder to form a lamination in which the sheet edges progress from inner to outer sheets to form the friction face of the pad. When laminated together, these stacks of sheets provide sections of the pad with different friction and wear characteristics which produces a pad which experiences generally uniform wear over its face during operation.
This new multi-laminate pad requires that several different friction material papers be developed and manufactured to make a single pad.
It would be desirable to provide a friction material pad in which different areas of the pad friction face have variable friction, wear rates, compressibility and stiffness (for noise attenuation) to produce a pad that is tailored to any specific applications.
It would also be desirable to provide such a friction material pad that does not require the use of several different papers all having different compositions.
It would be further desirable to provide such a friction material pad that exhibits generally uniform friction at all operating temperatures.